In recent years, digital still cameras for storing captured images as digital data have become popular. In a digital still camera, an image captured by optical lenses is photoelectrically converted into digital data by using an image-capturing device such as a CCD (Charge-Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) image sensor, after which predetermined signal processing is performed thereon, and the data is recorded on an external recording medium, etc.
As signal processing for a captured-image signal, automatic control, such as AE (Auto Exposure) for performing an appropriate exposure, AWB (Auto White Balance) for performing color correction according to the color temperature, and DCLP (Digital Clamp) for removing offset contained in the image signal, is performed. Detection for the above automatic control is performed based on the image signal which is read from the image-capturing device by thinning out pixels. Such an image signal is normally used as display data on an LCD (Liquid-Crystal Display) for monitoring during image capture by a user. Therefore, the reading operation mode that performs thinned-out reading is called a “monitor reading mode”.
In contrast, a reading operation mode that reads a signal from an image-capturing device without thinning out pixels is called a “frame reading mode” in the case of an interlace scanning method, and is called an “all-pixels reading mode” in the case of a progressive scanning method. These are collectively referred to as a “capture reading mode”.
FIG. 14 schematically shows an example of signal processing as the reading operation mode shifts in a conventional digital still camera.
FIG. 14 shows a case in which a CCD of an interlace scanning method is used as an image-capturing device. Part (A) of FIG. 14 shows a synchronization signal synchronized with the frame or the field. Part (B) of FIG. 14 shows the shift of the reading operation mode in the CCD. Parts (C) and (D) of FIG. 14 show the flow of a detection process and an image generation process for control of AE, AWB, and DCLP corresponding to the above shift, respectively. The detection process and the image generation process in parts (C) and (D) of FIG. 14 are performed by, for example, a camera block LSI.
At timings T1401 and T1402, as shown in part (B) of FIG. 14, the CCD operates in the monitor reading mode, and performs reading of signals, in which pixels are thinned out, in synchronization with the synchronization signal. The image signal obtained by the monitor reading mode is converted into digital data, after which, as shown in part (C) of FIG. 14, in the camera system LSI, detection for control of AE, AWB, and DCLP is performed. The data obtained by this detection is passed to, for example, a microcomputer, whereby coefficients for control of AE, AWB, and DCLP are computed, and these coefficients are output to the camera system LSI.
In the camera system LSI, as shown in part (D) of FIG. 14, based on the computed coefficients, an appropriate image-quality correction process is performed on the image signal obtained in the monitor reading mode, a process for conversion into a predetermined image data format is performed, and thus a process for generating an image signal for monitor (hereinafter referred to as a “monitor image signal”) is performed. The generated monitor image signal is stored in an image memory such as DRAM (Dynamic Random Access Memory), after which the monitor image signal is output to a display block, whereby it is displayed on a display device such as an LCD, so that monitoring by the user is performed.
The user adjusts the angle of view of the subject by using a display device, and the display image at this time requires a high frame rate. The monitor reading mode is a reading operation mode for generating a monitor image signal having a small amount of information by reading the signal by thinning out pixels in the CCD.
Next, at timing T1402, when, for example, a shutter switch is pressed by the user, the CCD is placed in the capture reading mode, and first, reading of signals is performed from all the pixels of the odd-numbered lines in the horizontal direction. At the subsequent timing T1403, reading of signals is performed from all the pixels of the even-numbered lines. In the capture reading mode, since pixels are not thinned out, the interval of the synchronization signal increases. The image signal obtained by the capture reading mode is converted into digital data, and this data is temporarily stored in the image memory.
Next, at timing T1404, the CCD returns to the monitor reading mode. Along with this, the image signal obtained by the capture reading mode is read from the image memory, this image signal is supplied to the camera system LSI, and a process for generating an image signal to be recorded on, for example, an external recording medium (hereinafter referred to as a “captured image signal”) is started. As shown in part (D) of FIG. 14, in the camera system LSI, by using various coefficients obtained from the image signal in the monitor reading mode prior to timing T1402 at which the shutter switch is pressed, correction processes, such as AE, AWB, and DCLP, for the image signal from the memory is performed by open control, and furthermore, a data conversion process is performed thereon, thereby generating a captured image signal. The generated captured image signal is again stored in the image memory, after which the signal is transferred to an external recording medium.
Thereafter at timing T1405, in the camera block LSI, the captured image signal generation process is terminated, and a detection for the image signal obtained in the monitor reading mode is started again.
As described above, in the conventional digital still camera, control of AE, AWB, and DCLP is performed on the image signal which is captured during the capture reading mode by using various coefficients obtained by detection in the monitor reading mode which is prior to the above capture reading mode. This results from being based on the concept that is constructed on the assumption that, conventionally, the camera signal processing and the detection process are performed in real time. That is, this is because, even if detection for the image signal which is output after the shutter is pressed is performed, the time to compute the data obtained by the detection using a microcomputer is required, and the camera signal processing cannot be performed in real time on the basis of this computed value.
Furthermore, in the conventional digital still camera, there is no large difference between the image signal which is read as a result of thinning out pixels in the monitor reading mode and the image signal which is read without thinning out pixels. Even if the detection is performed in the monitor reading mode, and a correction process is performed, based on open control, on the image signal obtained in the capture reading mode by using the various computed coefficients, a severe problem with the image quality of the captured image does not occur.
However, recently, since the number of pixels of the CCD has been increasing, the difference between the image signal obtained by the monitor reading mode and the image signal obtained by the capture reading mode has become large, and in the conventional method, correction processes of AE, AWB, and DCLP for the captured image cannot be performed appropriately.
For example, in the image-capturing device, the amount of a dark signal which occurs becomes larger according to the exposure time and the temperature, and as the number of pixels increases, the time it takes to read the image signal in the capture reading mode increases, causing noise components due to the dark signal generated in the image-capturing apparatus at that time to be increased. As a result, in the image signal obtained in each of the monitor reading mode and the capture reading mode, since the offsets which are contained therein differ greatly, in the image-quality correction process based on the detection in the monitor reading mode, a problem arises in that an appropriate image signal cannot be generated.
Furthermore, as the number of pixels increases, the ratio of thinning out pixels in the monitor reading mode must be increased so that the frame rate of the display image for monitoring by the user is not decreased. As a result, a conspicuous aliasing occurs at the space sampling on the image-capturing device. As one method for preventing this aliasing, a method of adding lines of the same color in the vertical direction on the image-capturing apparatus is becoming more common. However, in this method, for the image signal in the monitor reading mode, the amount of the output signal becomes two times as large due to the line addition. Therefore, during signal processing for the captured image, the process of correcting the captured image signal in which line addition is not performed is performed by using the data based on the detection of the image signal in which line addition is performed, thus making it difficult to perform proper control.
In addition to the problem involved with the increase of the number of pixels, in the correction process of AE, AWB, and DCLP, there are other conventional problems. For example, in the monitor reading mode, generally, an image is captured while the mechanical shutter is open. At this time, a so-called smear may occur in the signal output from the image-capturing device. In contrast, in the capture reading mode, a mechanical shutter is often used. In this case, a smear does not occur from the viewpoint of principles. Therefore, there can be cases in which a clear difference occurs between the image signals generated in the two reading operation modes.
Furthermore, a digital still camera often has a strobe mechanism installed therein. This strobe mechanism does not operate during monitor before an image is captured, that is, in the monitor reading mode, and operates only in the capture reading mode in which an image is captured. Therefore, based on the detection for the image signal in which strobe light is not emitted, the image signal in which strobe light is emitted is corrected, and it is impossible to perform accurate control from the viewpoint of principles.
The present invention has been made in view of such problems. An object of the present invention is to provide an image-capturing apparatus capable of appropriately performing image-quality correction on a captured image using solid-state image-capturing elements having a large number of pixels.
Another object of the present invention is to provide an image-quality correction method capable of suitably performing image-quality correction on a captured image using solid-state image-capturing elements having a large number of pixels.